/// <summary>
/// Uses Composition API to get the UIElement and sets an ExpressionAnimation
/// The ExpressionAnimation uses the height of the UIElement to calculate an opacity value
/// for the Header as it is scrolling off-screen. The opacity reaches 0 when the Header
/// is entirely scrolled off.
/// </summary>
/// <returns><c>true</c> if the assignment was successful; otherwise, <c>false</c>.</returns>
private bool AssignAnimation()
{
StopAnimation();
if (AssociatedObject == null)
{
return(false);
}
if (_scrollViewer == null)
{
_scrollViewer = AssociatedObject as ScrollViewer ?? AssociatedObject.FindDescendant <ScrollViewer>();
}
if (_scrollViewer == null)
{
return(false);
}
var listView = AssociatedObject as Windows.UI.Xaml.Controls.ListViewBase ?? AssociatedObject.FindDescendant <Windows.UI.Xaml.Controls.ListViewBase>();
if (listView != null && listView.ItemsPanelRoot != null)
{
Canvas.SetZIndex(listView.ItemsPanelRoot, -1);
}
if (_scrollProperties == null)
{
_scrollProperties = ElementCompositionPreview.GetScrollViewerManipulationPropertySet(_scrollViewer);
}
if (_scrollProperties == null)
{
return(false);
}
// Implicit operation: Find the Header object of the control if it uses ListViewBase
if (HeaderElement == null && listView != null)
{
HeaderElement = listView.Header as UIElement;
}
var headerElement = HeaderElement as FrameworkElement;
if (headerElement == null || headerElement.RenderSize.Height == 0)
{
return(false);
}
if (_headerVisual == null)
{
_headerVisual = ElementCompositionPreview.GetElementVisual(headerElement);
}
if (_headerVisual == null)
{
return(false);
}
_scrollViewer.ViewChanged -= ScrollViewer_ViewChanged;
_scrollViewer.ViewChanged += ScrollViewer_ViewChanged;
_scrollViewer.GotFocus -= ScrollViewer_GotFocus;
_scrollViewer.GotFocus += ScrollViewer_GotFocus;
headerElement.SizeChanged -= ScrollHeader_SizeChanged;
headerElement.SizeChanged += ScrollHeader_SizeChanged;
var compositor = _scrollProperties.Compositor;
if (_animationProperties == null)
{
_animationProperties = compositor.CreatePropertySet();
_animationProperties.InsertScalar("OffsetY", 0.0f);
}
var propSetOffset = _animationProperties.GetReference().GetScalarProperty("OffsetY");
var scrollPropSet = _scrollProperties.GetSpecializedReference <ManipulationPropertySetReferenceNode>();
var expressionAnimation = ExpressionFunctions.Max(ExpressionFunctions.Min(propSetOffset, -scrollPropSet.Translation.Y), 0);
_headerVisual.StartAnimation("Offset.Y", expressionAnimation);
return(true);
}
/// <summary>
/// Uses Composition API to get the UIElement and sets an ExpressionAnimation
/// The ExpressionAnimation uses the height of the UIElement to calculate an opacity value
/// for the Header as it is scrolling off-screen. The opacity reaches 0 when the Header
/// is entirely scrolled off.
/// </summary>
/// <returns><c>true</c> if the assignment was successful; otherwise, <c>false</c>.</returns>
private bool AssignAnimation()
{
StopAnimation();
// Confirm that Windows.UI.Xaml.Hosting.ElementCompositionPreview is available (Windows 10 10586 or later).
if (!ApiInformation.IsMethodPresent("Windows.UI.Xaml.Hosting.ElementCompositionPreview", nameof(ElementCompositionPreview.GetScrollViewerManipulationPropertySet)))
{
// Just return true since it's not supported
return(true);
}
if (AssociatedObject == null)
{
return(false);
}
if (_scrollViewer == null)
{
_scrollViewer = AssociatedObject as ScrollViewer ?? AssociatedObject.FindDescendant <ScrollViewer>();
}
if (_scrollViewer == null)
{
return(false);
}
if (_scrollProperties == null)
{
_scrollProperties = ElementCompositionPreview.GetScrollViewerManipulationPropertySet(_scrollViewer);
}
if (_scrollProperties == null)
{
return(false);
}
// Implicit operation: Find the Header object of the control if it uses ListViewBase
if (HeaderElement == null)
{
var listElement = AssociatedObject as Windows.UI.Xaml.Controls.ListViewBase ?? AssociatedObject.FindDescendant <Windows.UI.Xaml.Controls.ListViewBase>();
if (listElement != null)
{
HeaderElement = listElement.Header as UIElement;
}
}
var headerElement = HeaderElement as FrameworkElement;
if (headerElement == null || headerElement.RenderSize.Height == 0)
{
return(false);
}
if (_headerVisual == null)
{
_headerVisual = ElementCompositionPreview.GetElementVisual(headerElement);
}
if (_headerVisual == null)
{
return(false);
}
_scrollViewer.ViewChanged -= ScrollViewer_ViewChanged;
_scrollViewer.ViewChanged += ScrollViewer_ViewChanged;
_scrollViewer.GotFocus -= ScrollViewer_GotFocus;
_scrollViewer.GotFocus += ScrollViewer_GotFocus;
headerElement.SizeChanged -= ScrollHeader_SizeChanged;
headerElement.SizeChanged += ScrollHeader_SizeChanged;
var compositor = _scrollProperties.Compositor;
if (_animationProperties == null)
{
_animationProperties = compositor.CreatePropertySet();
_animationProperties.InsertScalar("OffsetY", 0.0f);
}
var propSetOffset = _animationProperties.GetReference().GetScalarProperty("OffsetY");
var scrollPropSet = _scrollProperties.GetSpecializedReference <ManipulationPropertySetReferenceNode>();
var expressionAnimation = ExpressionFunctions.Max(ExpressionFunctions.Min(propSetOffset, -scrollPropSet.Translation.Y), 0);
_headerVisual.StartAnimation("Offset.Y", expressionAnimation);
return(true);
}
